CA2026902C - Process for nitrocarburizing components made from steel - Google Patents

Abstract

Components made from steel can be nitrocarburized in cyanate- and cyanide-containing salt baths within relatively short times if the process is carried out at 600 to 700°C in a first stage and at 560 to 590°C in a second stage. The components exhibit the same or better characteristics as with considerably longer processing at 560 to 590°C.

Description

CA 02026902 1998-02-2~

Process For Nitrocarburizing Components Made From Steel The invention relates to a process for nitrocarburizing components made from steel in cyante- and cyanide-containing salt melts at temperatures between 560 to 700~C.
Of particular interest is the application of the process to structural elements.

For nitrocarburizing parts made from iron and steel use is made predominantly of salt melts consisting of mixtures of alkaline cyanides, alkaline cyanates and alkaline carbonates, generally maintained at temperatures between 560 and 590~C. Also known are nitrocarburizing salt baths in which processing takes place at higher temperatures, up to 700~C.

The workpieces are subjected for some time to the action of the salt melt, whereby nitrogen and carbon are diffused into the workpieces and iron carbon nitride phases are formed within the surface layers. These phases increase, in particular, the wear resistance and corrosion resistance of the components. The nitrogen and carbon available in the salt melt, the processing temperature, and the processing time all influence the structure, the thickness and the quality of the nitrocarburized layer in the steel.

If the nitrocarburizing process is carried out in the cyanate- and cyanide-containing salt baths, for instance at 580~C, components are produced which exhibit good wear and corrosion resistance. This is due to the formation of the monophase layer of E-iron carbonitride. At 580~C, relatively long periods of time, as a rule several hours, are required for a layer of adequate thickness and quality to form.

't CA 02026902 1998-02-2~

~. _ At higher temperatures, for instance at 630~C, E-layers of iron carbonitride of appropriate thickness are produced more quickly, however, the wear resistance and the corrosion resistance of the components thus processed is definitely lower than with components processed at 580~C in identical salt melts.

It is an object of the present invention to provide a process for nitrocarburizing components made from steel in cyanate- and cyanide-containing salts melts at temperatures between 560 and 700~C, so designed as to ensure that the components exhibit good wear and corrosion characteristics after a minimum processing time.

According to this invention, this object and other objects are achieved by carrying out the nitrocarburizing process in a first stage at 600 to 700~C., and in a second stage at 560 to 590~C, wherein the ratio between nitrocarburizing times of the two stages falls within a range from 20:1 to 1:15.

With a preferred method, the process is carried out in the first stage at 610 to 650~C during a period of 10 to 120 minutes, and in the second stage at 570 to 590~C during a period of 10 to 90 minutes.

With this two-stage process an adequately thick interface is achieved, whereby, surprisingly, the interface processes the same characteristics as an interface formed during considerably longer periods of time at temperatures from 560 to 590~C. It is possible as a result to reduce the nitrocarburizing time considerably, without impairing the quality of the components processed as regards wear, corrosion, fatigue strength, etc.

CA 02026902 1998-02-2~

Figures 1 and 2 illustrate by way of graphs the corrosion characteristics (Fig. 1) and the wear resistance (Fig. 2) of specimens made from steel CK 45 after processing, at different temperatures, in a salt melt containing 2.8 wt.%
cyanide and 37.4 wt.% cyanate.

Fig. 1 compares the corrosion resistance of cylindrical specimens after nitrocarburizing. In all cases the process was carried out in such a way as to achieve an interface of about 15 ~m thickness. The column on the left shows the corrosion resistance of piston rods nitrocarburized for 90 minutes at 580~C. The column in the middle shows the resistance of the components after nitrocarburizing for 30 minutes at 630~C, and the column on the right shows the resistance of specimens nitrocarburized initially for 20 minutes at 630~C and subsequently for 20 minutes at 580~C.

The wear characteristics also were favourably affected by the two-stage process according to the invention. The result of an Amsler test is shown in Figure 2. Curve 1 corresponds to the normal processing, for 90 minutes at 580~C, of specimens made from the material C45; curve 2 relates to specimens nitrocarburized for 30 minutes at 630~C, and curve 3 shows the result of two-stage nitrocarburizing for 20 minutes at 630~C and for 20 minutes at 580~C. The wear characteristics of a component processed for 20 minutes at 630~C and then for 20 minutes at 580~C are therefore comparable with those of a component nitrocarburized for 90 minutes at 580~C. It follows that the process according to the invention reduces the processing time by more than half.

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Claims (2)

1. A process for nitrocarburizing a component made at least in part from steel in a cyanate- and cyanide-containing salt melt at a temperature between 560° and 700°C., the process comprising immersing the component in the melt in a first stage at 600° to 700°C., and in a second stage at 560° to 590°C., wherein the ratio of time of immersion in said first stage to time of immersion in said second stage is from 20:1 to 1:15.

2. A process for nitrocarburizing a component made at least in part from steel in a cyanate- and cyanide-containing salt melt at a temperature between 560° and 700°C., the process comprising immersing the component in the melt in a first stage at 610° to 650°C. for 10 to 120 minutes, and in a second stage at 570° to 590°C. for 10 to 90 minutes.